STL, an acronym for STereoLithography; and a kind of file

1. The format

Files in .stl format are widely used for the description of the shapes of
solid bodies.

The nature of the format is fully described at this
website,
from which a single summarising paragraph has been extracted, as follows:

"An StL (StereoLithography) file is a triangular representation of a
3-dimensional surface geometry.

" The surface is tessellated or broken down logically into a series of
small triangles (facets).

"Each facet is described by a perpendicular direction and three points
representing the vertices (corners) of the triangle."

2. The use of STL files by PHOENICS

When PHOENICS introduces an object into its 'virtual-reality' editor,
one of the permitted formats of file which defines the shape of the object
is STL.

At first, only ASCII STL files were supported; however, from PHOENICS
version 3.6, binary STL files are also allowed.

There exists a directory within the PHOENICS package which contains
a set of ASCII STL files, of which the names can be seen by
clicking here.

One of these has been copied into a .htm file, and provided with the necessary
HTM tags, in order that its contents can be inspected, and the easy
readability of its format perceived, by clicking
here.

3. STL files created by PHOENICS

PHOENICS can not only use files created by other software packages:
it can create them itself.

This facility has been provided in order that:

the shape and quality of the body actually simulated by the
PARSOL
feature of PHOENICS can be inspected; and

the STL file produced by a CAD package may be replaced by one with
fewer too-far-from-equilateral triangles than the original possessed,
and therefore easier to use as the basis of grid generation.

The first of these is the more important for PHOENICS users; and its
value may be seen from the following sequence of images, concerned with
the use of PARSOL to secure an adequate representation of flow through
an array of louvres.

Click here
to see a disappointingly unrealistic flow pattern, achieved with a grid
of 46*75.

Click here
to see the explanation: the first attempt at 'parsolization' has made a
very poor (grey) representation of the body (green) defined by the
original STL file,
because the grid was too coarse. It was activation of the MAKSTL feature
of PHOENICS which showed this.

The grid was therefore refined appreciably, namely to: 120*135 ;
this resulted in the flow seen by clicking
here, where both
the body shape and the flow are seen to be smooth.

How STL-making is activated

The user sets MAKSTL=T in the q1 file and PARSOL=T
(which is the default)

This causes the PHOENICS solver, EARTH, to create an stl file for each
VR-object.

The user can also set STLONL=T, which will
cause EARTH to create the STL files, without proceeding further to
perform the CFD calculation. This reduces the memory requirement
during the PARSOL calculations, allowing larger problems to be handled.

4. Other STL support.

The general-purpose geometry-creation utility
AC3D
can also be used to read, edit or display STL files (both ascii and binary).

It is sometimes useful to be able to confirm the shape an object and
integrity of the source file with this independent program.